On December 5th 1952, a dense and deadly smog settled over London.

Called the Great Smog, the event was triggered by the burning of coal for electricity and heat during a prolonged calm and cold spell, that left a thick layer of smog over the capital for four days.

Smoke would normally rise into the atmosphere and disperse, but high pressure was hanging overhead in December of 1952, similar to what we've experienced recently.

A high pressure system, or anticyclone, pushes warm air downwards and creates a temperature inversion, so that the air closest to the ground is cooler than the air above it.

On the morning of the 5th, conditions were ideal for radiation fog to form, under clear skies and light winds. This meant that all the smoke then became trapped at the surface in a thick layer of fog.

It is believed that more than 4,000 people died directly from the event - although more recent research puts this figure at far higher in the realms of 12,000 deaths - , with more than 100,000 people likely having respiratory illness.

The Great Smog is said to be the worst air pollution event of in the UK's history. It led to changes in practices and regulations for air quality and health with the introduction of the Clean Air Act in 1956.